As global warming causes glaciers to melt, rivers lose their diversity of plants and animals at an alarming rate, scientists warn.

In a four-year experiment simulating the effects of glacier retreat, a team of scientists from Ecuador and France found that the abundance of species of Ecuador’s rivers changed 30 times faster than their rate of recovery when original conditions were restored.

When the water runoff from the glacier decreased, non-native seaweed and herbivorous insects proliferated at the expense of native species, the researchers say in a paper published in Nature Communications last month (24 June).

The scientists studied two streams located at 4,100 metres above sea level in Ecuador’s Antisana Ecological Reserve. One was used as a reference and the other one was manipulated to mimic the effects of a shrinking glacier on the downstream river’s flow, its plants and animals.

“It’s quite a difficult experiment that had never been done before,” says lead author Sophie Cauvy-Fraunié, an ecologist at France’s Institute of Research for Development.

During the first year, the team monitored the river’s species to establish baseline conditions.

At the beginning of the second year, the scientists built a stone dam to divert about a third of the river’s water flow, simulating the effects of reduced water runoff due to the glacier’s shrinkage. They then compared the biodiversity of the manipulated stream with that of the reference stream.

“We don’t have the same community during and after the manipulation,” Cauvy-Fraunié tells SciDev.Net. We see a “brutal change” characterised by a rise in seaweed and herbivorous species such as insects, she says.

After just two weeks of reduced flow, the scientists observed a proliferation of seaweed, which returned to its initial density 20 months after the original flow was restored, during the experiment’s third year.

The herbivore biomass also shot up two weeks after the river flow was reduced, and took 14 months to return to its initial level.

Matthias Bernet, a geologist at the University of Grenoble in France, says the experiment provides interesting insights into the speed of ecosystem changes when glacier water flows change.

But the study actually simulates glacier disappearance rather than shrinkage, Bernet says.

Cauvy-Fraunié explains that rising temperatures will initially accelerate glacier melting and produce more water — but, after a certain time, the water will decrease when its source disappears altogether.

Native species will have to withstand different environmental conditions and face competition with other species, she says. “The flow will decrease and, unfortunately, some species will disappear.”